PROJECT SUMMARY/ABSTRACT Autosomal dominant polycystic kidney disease (PKD) is characterized by the accumulation of numerous renal cysts leading to a progressive decline in kidney function which frequently culminates in end-stage renal disease (ESRD). Aberrant tubular epithelial cell proliferation, macrophage infiltration, and tubulointerstitial fibrosis are important contributors to PKD progression; however, factors promoting these events remain undetermined. Systemic phosphate balance is tightly regulated, with renal excretion of phosphate being critically important for the elimination of excess dietary phosphate. When functional nephron numbers are reduced, as in PKD, phosphate excretion by residual nephrons is dramatically increased to preserve phosphate balance. Fibroblast growth factor 23 (FGF23), a circulating hormone that induces urinary phosphate excretion, is elevated in early- stage PKD and is primarily responsible for this maintenance of phosphate balance by the kidneys. Both high dietary phosphate consumption and elevated FGF23 are associated with a more rapid decline in renal function in chronic kidney disease (CKD), indicating that increased urinary phosphate excretion may contribute to decrements in kidney function. We propose that high concentrations of tubular phosphate are nephrotoxic, leading to progressive kidney injury, immune cell infiltration, and fibrosis. In preliminary studies, we observed phosphaturic mouse models to exhibit evidence of early kidney injury and fibrosis. Moreover, additional experiments revealed dietary phosphate restriction slows PKD progression in several mouse models of cystic kidney disease. The kidney expression of osteopontin (OPN), a matricellular protein that is produced by tubular epithelial cells and uses an ASARM peptide motif to enhance phosphate solubility in urine, is increased in rodent models of both phosphaturia and cystic kidney disease. In addition to its function of inhibiting mineral aggregation, OPN has established functions to stimulate cell proliferation and enhance macrophage recruitment to sites of injury; thus, kidney OPN production in PKD may contribute to the pathophysiology of cyst formation and associated pathology. We hypothesize that high concentrations of tubular phosphate in PKD contribute to epithelial cell injury and OPN production, which together promote cyst epithelial cell proliferation and macrophage recruitment that accelerate kidney disease progression. We will use the proposed experiments to determine: (1) the impact of high urine phosphate on PKD progression, (2) the role of osteopontin in mediating the effect of high dietary phosphate on cyst growth, macrophage recruitment and interstitial fibrosis, (3) if administration of a decoy ASARM peptide can enhance the solubility of tubular phosphate and prevent PKD progression, and (4) the prevalence, composition, and surrounding pathology of mineral aggregates in human and mouse PKD kidneys.